A known brake system for a vehicle is disclosed in U.S. Pat. No. 5,000,520. The disclosed vehicle brake system includes a master cylinder and an auxiliary hydraulic pressure source. The master cylinder is a hydraulic pressure generating device for generating brake hydraulic pressure in response to operation of a brake pedal. The auxiliary hydraulic pressure source is provided with a hydraulic pressure pump and an accumulator. Power hydraulic pressure of the auxiliary hydraulic pressure source is controlled by a pressure switch connected to the auxiliary hydraulic pressure source. The pressure switch is closed when power hydraulic pressure of the auxiliary hydraulic pressure source reaches a lower threshold pressure value and is opened when power hydraulic pressure of the auxiliary hydraulic pressure source reaches an upper threshold pressure value.
In the vehicle brake system mentioned above, a predetermined difference between the upper threshold pressure value and the lower threshold pressure value may need to be maintained so as not to cause fluctuated operation of the pressure switch due to hysteresis of the pressure switch. Therefore, power hydraulic pressure of the auxiliary hydraulic pressure source may be varied between the upper threshold pressure value and the lower threshold pressure value.
Further, according to the vehicle brake system mentioned above, when the vehicle brake system malfunctions, for example when a contact of a relay for a switching operation of an electric motor malfunctions, an accumulator hydraulic pressure can be maintained within a predetermined pressure level by a pressure switch and a relay. In this case, the accumulator hydraulic pressure corresponds to the power hydraulic pressure outputted from the auxiliary hydraulic pressure source. Further, when the accumulator hydraulic pressure is decreased and falls below the lower threshold pressure value, a malfunction signal is sent by the other pressure switch. Therefore, the electric motor can be driven by the other relay.
However, one of the known accumulators for the above-disclosed brake system is an accumulator which is provided with a cylinder and a piston slidably disposed in the cylinder. Nitrogen gas is sealed in a gas sealed chamber defined between the cylinder and the piston. When the accumulator hydraulic pressure is increased, the gas sealed chamber is compressed and is further maintained within a predetermined pressure range. The above described accumulator requires a sealing mechanism for sealing the gas sealed chamber.
Recent developments have lead to a down-sizing of the accumulator. To meet this demand, it is necessary to accurately detect the fluid accumulated in the accumulator. Further, a desired amount of fluid is required to be accumulated in the accumulator. Therefore, the ON-OFF control by the known pressure switch is replaced with a pressure sensor which can consecutively detect the accumulator hydraulic pressure. The pressure sensor detects the accumulator hydraulic pressure and estimates the amount of fluid accumulated in the accumulator based upon the detected hydraulic pressure value. However, with the accumulator having the above-described structure, fluid may not be accumulated in the accumulator corresponding to the pressure increase.
Accordingly, the above-described vehicle brake systems are susceptible of certain improvements with respect to having the auxiliary hydraulic pressure source provided with the hydraulic pressure pump and the accumulator. The hydraulic pressure pump needs to be properly controlled based upon the accumulator hydraulic pressure monitored consecutively or continuously so that a predetermined amount of fluid can be accumulated in the accumulator.
Another known brake system for a vehicle is disclosed in published Japanese Patent Application Toku-Kai-Hei 8-268256. The power hydraulic pressure of a hydraulic pressure pump is controlled in response to the sum of the brake controlling pressure based upon a vehicle driving condition and margin pressure based upon brake control. According to the above-described vehicle brake system, brake fluid supplied by the hydraulic pressure pump may fluctuate when a brake operation is repeatedly performed.
Accordingly, the disclosed vehicle brake system is susceptible of certain improvements with respect to providing an auxiliary hydraulic pressure source for supplying brake hydraulic pressure which is less variable.
An auxiliary hydraulic pressure source device for a vehicle according to one aspect of the invention includes a hydraulic pressure pump for pressurizing and discharging fluid, an accumulator for accumulating the fluid pressurized and discharged by the hydraulic pressure pump, hydraulic pressure detecting means for continuously detecting the accumulator hydraulic pressure, and control means for controlling operation of the hydraulic pressure pump in accordance with a comparison result obtained by comparing the detection result of the hydraulic pressure detecting means with a predetermined target pressure. The control means includes a pressure difference calculating means for calculating a pressure difference between the detection result of the hydraulic pressure detecting means and the predetermined target pressure, a pressure change calculating means for calculating a pressure change of the accumulator hydraulic pressure per unit time based upon the detection result of the hydraulic pressure detecting means, and adjusting means for adjusting an accumulating operation of the fluid accumulated in the accumulator by the operation of the hydraulic pressure pump based on the pressure difference and the pressure change of the accumulator hydraulic pressure per unit time.
An auxiliary hydraulic pressure source device for a vehicle according to another aspect of the invention includes a hydraulic pressure pump for pressurizing and discharging fluid, an accumulator for accumulating the fluid pressurized and discharged by the hydraulic pressure pump, a hydraulic pressure detecting means for detecting the accumulator hydraulic pressure continuously, and control means for controlling operation of the hydraulic pressure pump in accordance with a comparison result obtained by comparing the detection result of the hydraulic pressure detecting means with a predetermined pump-drive-starting reference pressure and a predetermined pump-drive-stopping reference pressure. The control means includes pressure change calculating means for calculating a pressure change of the accumulator hydraulic pressure per unit time based upon the detection result of the hydraulic pressure detecting means, comparing means for comparing the calculation result of the pressure change calculating means with a predetermined value, and adjusting means for adjusting an accumulating operation of the fluid accumulated in the accumulator by the operation of the hydraulic pressure pump in accordance with the comparison result of the comparing means.
According to another aspect of the invention, a method for controlling power hydraulic pressure supplied to an accumulator involves pressurizing and discharging fluid through operation of a hydraulic pump and accumulating the pressurized and discharged fluid in an accumulator, continuously detecting an accumulator hydraulic pressure in the accumulator, controlling operation of the hydraulic pressure pump based on a comparison of the detected accumulator hydraulic pressure with a predetermined target pressure, calculating a pressure difference between the detected accumulator hydraulic pressure and the predetermined target pressure, calculating a pressure change of the accumulator hydraulic pressure per unit time based upon the detected accumulator hydraulic pressure, and adjusting the fluid accumulated in the accumulator through operation of the hydraulic pressure pump based on the calculated pressure difference and the calculated pressure change of the accumulator hydraulic pressure per unit time.